Then I’m off to London watchin' this bridge that we built destroy

from XV – Smallville Lyrics on Rap Genius

Meaning

It looks like he has lost a battle or link with something/someone (i.e. the burning of a bridge). To emphasize the point, he references one of the grandest and most iconic bridges in the world, the London Bridge. Said bridge has been destroyed and rebuilt many a time, which ties well with this line.

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